Methods and systems for determining average oxidation state of redox flow battery systems

ABSTRACT

A method for determining an average oxidation state (AOS) of a redox flow battery system includes measuring a charge capacity for a low potential charging period starting from a discharged state of the redox flow battery system to a turning point of a charge voltage; and determining the AOS using the measured charge capacity and volumes of anolyte and catholyte of the redox flow battery system. Other methods can be used to determine the AOS for a redox flow battery system or use discharge voltage instead of charging voltage.

RELATED PATENT APPLICATIONS

The present patent application claims the benefit of U.S. ProvisionalPatent Application Ser. No. 62/849,959, filed May 20, 2019, incorporatedherein by reference in its entirety.

FIELD

The present invention is directed to the area of redox flow batterysystems and methods of making and using redox flow battery systems. Thepresent invention is also directed to methods and systems fordetermining average oxidation state of redox flow battery systems andrebalancing redox flow battery systems.

BACKGROUND

The cost of renewable power generation has reduced rapidly in the pastdecade and continues to decrease as more renewable power generationelements, such as solar panels, are deployed. However, renewable powersources, such as solar, hydroelectric, and wind sources, are oftenintermittent and the pattern of user load does not typically coincidewith the intermittent nature of the sources. There is a need for anaffordable and reliable energy storage system to store power generatedby renewable power sources when available and to provide power to userswhen there is insufficient power generation from the renewable powersources.

BRIEF SUMMARY

One embodiment is a method for determining an average oxidation state(AOS) of a redox flow battery system. The method includes measuring acharge capacity for a low potential charging period starting from adischarged state of the redox flow battery system to a turning point ofa charge voltage; and determining the AOS using the measured chargecapacity and volumes of anolyte and catholyte of the redox flow batterysystem.

In at least some embodiments, the anolyte and the catholyte of the redoxflow battery system both includes chromium ions and iron ions anddetermining the AOS further includes determining the AOS using initialconcentrations of chromium ions and iron ions in the anolyte andcatholyte. In at least some embodiments, a molar ratio of chromium inthe anolyte to iron in the catholyte is at least 1.25. In at least someembodiments, the low potential charging period corresponds to a periodof reduction of Fe³⁺ in the anolyte.

In at least some embodiments, the method further includes rebalancingthe AOS in response to the determination. In at least some embodiments,rebalancing the AOS includes oxidizing vanadium ions in a balancingelectrolyte to dioxovanadium ions to produce hydrogen ions, wherein theanolyte or catholyte of the redox flow battery system form a balancingarrangement with the balancing electrolyte using at least twohalf-cells; and regenerating the vanadium ions by reducing thedioxovanadium ions using a reductant.

In at least some embodiments, the method further includes, prior tomeasuring the charge capacity, applying a potential to discharge theredox flow battery system. In at least some embodiments, measuring thecharge capacity includes measuring the charge capacity in the anolytewithin the redox flow battery system. In at least some embodiments,measuring the charge capacity includes measuring the charge capacity ina portion of the anolyte removed from the redox flow battery system.

Another embodiment is a method for determining an average oxidationstate (AOS) of a redox flow battery system including a catholyte and ananolyte that both include iron and chromium ions. The method includesmeasuring in situ an amount or concentration of a first ionic species ina first one of either the catholyte or the anolyte; measuring in situ anamount or concentration of a second ionic species in a second one ofeither the catholyte or the anolyte; and determining the AOS using themeasured amounts or concentrations, initial concentrations or amounts ofiron and chromium in the catholyte and anolyte, and volumes of anolyteand catholyte of the redox flow battery system.

In at least some embodiments, a molar ratio of chromium in the anolyteto iron in the catholyte is at least 1.25. In at least some embodiments,the method further includes rebalancing the AOS in response to thedetermination. In at least some embodiments, rebalancing the AOSincludes oxidizing vanadium ions in a balancing electrolyte todioxovanadium ions to produce hydrogen ions, wherein the anolyte orcatholyte of the redox flow battery system form a balancing arrangementwith the balancing electrolyte using at least two half-cells; andregenerating the vanadium ions by reducing the dioxovanadium ions usinga reductant.

In at least some embodiments, measuring in situ the amount orconcentration of the first ionic species includes measuring the amountor concentration of the first ionic species with the first one of eitherthe catholyte or the anolyte not flowing and another one of thecatholyte or anolyte flowing. In at least some embodiments, measuring insitu the amount or concentration of the second ionic species comprisesmeasuring the amount or concentration of the second ionic species withthe second one of either the catholyte or the anolyte not flowing andanother one of the catholyte or anolyte flowing.

In at least some embodiments, measuring in situ the amount orconcentration of the first ionic species includes measuring the amountor concentration of the first ionic species by electrochemicallytitrating the first ionic species using vanadium ions in a balancingelectrolyte.

Yet another embodiment is a method for determining an average oxidationstate (AOS) of a redox flow battery system including a catholyte and ananolyte that both include iron and chromium ions. The method includesmeasuring a discharge rate during discharge of a redox flow batterysystem; determining a turning point of the discharge and measuring anopen circuit voltage (OCV) after the turning point; estimating aconcentration of Fe' using the measured OCV; and determining the AOSusing the measured concentration and volumes of anolyte and catholyte ofthe redox flow battery system.

In at least some embodiments, a molar ratio of chromium in the anolyteto iron in the catholyte is at least 1.25. In at least some embodiments,the method further includes rebalancing the AOS in response to thedetermination. In at least some embodiments, rebalancing the AOSincludes oxidizing vanadium ions in a balancing electrolyte todioxovanadium ions to produce hydrogen ions, wherein the anolyte orcatholyte of the redox flow battery system form a balancing arrangementwith the balancing electrolyte using at least two half-cells; andregenerating the vanadium ions by reducing the dioxovanadium ions usinga reductant.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of one embodiment of a redox flow batterysystem, according to the invention;

FIG. 2 is a schematic diagram of one embodiment of an electrode for aredox flow battery system, according to the invention;

FIG. 3 is a flowchart of a one embodiment of removing or reducingimpurities in a redox flow battery system, according to the invention;

FIG. 4 is a schematic diagram of another embodiment of a redox flowbattery system with the catholyte diverted into the second half-cell formaintenance, according to the invention;

FIG. 5A is a schematic diagram of one embodiment of a system thatincludes a redox flow battery system in conjunction with a balancingarrangement, according to the invention;

FIG. 5B is a schematic diagram of one embodiment of the balancingarrangement of the system of FIG. 5A, according to the invention;

FIG. 5C is a schematic diagram of another embodiment of a system thatincludes a redox flow battery system in conjunction with a balancingarrangement, according to the invention;

FIG. 5D is a schematic diagram of one embodiment of the balancingarrangement of the system of FIG. 5C, according to the invention;

FIG. 5E is a schematic diagram of another embodiment of a balancingarrangement, according to the invention;

FIG. 6A is a schematic diagram of electrolyte tanks of a redox flowbattery system with pressure release valves, according to the invention;

FIG. 6B is a schematic diagram of an electrolyte tank of a redox flowbattery system with a liquid-containing U-tube arrangement for pressurerelief, according to the invention;

FIG. 6C is a schematic diagram of electrolyte tanks of a redox flowbattery system with an arrangement for migration of gas between thetanks, according to the invention;

FIG. 7 is a schematic diagram of another embodiment of a redox flowbattery system with a secondary container, according to the invention;

FIG. 8 is a schematic diagram of another embodiment of a redox flowbattery system with a temperature zone, according to the invention;

FIG. 9 is a graph of charge capacity versus charge voltage for onebalanced redox flow battery system and two unbalanced redox flow batterysystems, according to the invention;

FIG. 10 is a flowchart of one embodiment of a method of determining anaverage oxidation state (AOS) of a redox flow battery system, accordingto the invention;

FIG. 11 is a flowchart of another embodiment of a method of determiningan average oxidation state (AOS) of a redox flow battery system,according to the invention;

FIG. 12 is a graph of self-discharge time versus discharge voltage foran unbalanced redox flow battery system, according to the invention;

FIG. 13 is a flowchart of a third embodiment of a method of determiningan average oxidation state (AOS) of a redox flow battery system,according to the invention;

FIG. 14 is a schematic diagram of yet another embodiment of a redox flowbattery system and includes a cell for measuring an open circuit voltage(OCV), according to the invention;

FIG. 15 is a flowchart of one method of determining storage or chargecapacity by measurement of an end OCV, according to the invention; and

FIG. 16 is a flowchart of one method of determining AOS using ameasurement of OCV, according to the invention.

DETAILED DESCRIPTION

The present invention is directed to the area of redox flow batterysystems and methods of making and using redox flow battery systems. Thepresent invention is also directed to iron-chromium (Fe—Cr) redox flowbattery systems and methods of making an using Fe—Cr redox flow batterysystems.

Redox flow battery systems are a promising technology for the storage ofenergy generated by renewable energy sources, such as solar, wind, andhydroelectric sources, as well as non-renewable and other energysources. As described herein, in at least some embodiments, a redox flowbattery system can have one or more of the following properties: longlife; reusable energy storage; or tunable power and storage capacity.The terms “storage capacity”, “energy capacity”, and “charge capacity”are used interchangeably unless otherwise indicated.

FIG. 1 illustrates one embodiment of a redox flow battery system 100. Itwill be recognized that other redox flow battery systems 100 may includemore or fewer elements and the elements may be arranged differently thanshown in the illustrated embodiments. It will also be recognized thatthe description below of components, methods, systems, and the like canbe adapted to other redox flow battery systems different from theillustrated embodiments.

The redox flow battery system 100 of FIG. 1 includes two electrodes 102,104 and associated half-cells 106, 108 that are separated by a separator110. The electrodes 102, 104 can be in contact or separated from theseparator. Electrolyte solutions flow through the half-cells 106, 108and are referred to as the anolyte 112 and the catholyte 114. The redoxflow battery system 100 further includes an anolyte tank 116, acatholyte tank 118, an anolyte pump 120, a catholyte pump 122, ananolyte distribution arrangement 124, and a catholyte distributionarrangement 126. The anolyte 112 is stored in the anolyte tank 116 andflows around the anolyte distribution arrangement 124 through, at leastin part, action of the anolyte pump 120 to the half-cell 106. Thecatholyte 114 is stored in the catholyte tank 118 and flows around thecatholyte distribution arrangement 126 through, at least in part, actionof the catholyte pump 122 to the half-cell 108. It will be recognizedthat, although the illustrated embodiment of FIG. 1 includes a singleone of each of the components, other embodiments can include more thanone of any one or more of the illustrated components. For example, otherembodiments can include multiple electrodes 102, multiple electrodes104, multiple anolyte tanks 116, multiple catholyte tanks 118, multiplehalf-cells 112, or multiple half-cells 114, or any combination thereof.

The anolyte and the catholyte are electrolytes and can be the sameelectrolyte or can be different electrolytes. During energy flow into orout of the redox flow battery system 100, the electrolyte in one of thehalf-cells 106, 108 is oxidized and loses electrons and the electrolytein the other one of the half-cells is reduced and gains electrons.

The redox flow battery system 100 can be attached to a load/source130/132, as illustrated in FIG. 1. In a charge mode, the redox flowbattery system 100 can be charged or recharged by attaching the flowbattery to a source 132. The source 132 can be any power sourceincluding, but not limited to, fossil fuel power sources, nuclear powersources, other batteries or cells, and renewable power sources, such aswind, solar, or hydroelectric power sources. In a discharge mode, theredox flow battery system 100 can provide energy to a load 130. In thecharge mode, the redox flow battery system 100 converts electricalenergy from the source 132 into chemical potential energy. In thedischarge mode, the redox flow battery system 100 converts the chemicalpotential energy back into electrical energy that is provided to theload 130.

The redox flow battery system 100 can also be coupled to a controller128 that can control operation of the redox flow battery system. Forexample, the controller 128 may connect or disconnect the redox flowbattery system 100 from the load 130 or source 132. The controller 128may control operation of the anolyte pump 120 and catholyte pump 122.The controller 128 may control operation of valves associated with theanolyte tank 116, catholyte tank 118, anolyte distribution system 124,catholyte distribution system 126, or half-cells 106, 108. Thecontroller 128 may be used to control general operation of the redoxflow battery system 100 include switching between charge mode, dischargemode, and, optionally, a maintenance mode (or any other suitable modesof system operation.) In at least some embodiments, the controller orthe redox flow battery system may control the temperature of within thehalf-cells or elsewhere in the system. In at least some embodiments, thetemperature of the half-cells (or the system in general or portions ofthe system) is controlled to be no more than 65, 60, 55, or 50 degreesCelsius during operation.

Any suitable controller 128 can be used including, but not limited to,one or more computers, laptop computers, servers, any other computingdevices, or the like or any combination thereof and may includecomponents such as one or more processors, one or more memories, one ormore input devices, one or more display devices, and the like. Thecontroller 128 may be coupled to the redox flow battery system throughany wired or wireless connection or any combination thereof. Thecontroller 128 (or at least a portion of the controller) may be locatedlocal to the redox flow battery system 100 or located, partially orfully, non-locally with respect to the redox flow battery system.

The electrodes 102, 104 can be made of any suitable material including,but not limited to, graphite or other carbon materials (including solid,felt, paper, or cloth electrodes made of graphite or carbon), gold,titanium, lead, or the like. The two electrodes 102, 104 can be made ofthe same or different materials. In at least some embodiments, the redoxflow battery system 100 does not include any homogenous or metalliccatalysts for the redox reaction in the anolyte or catholyte or both.This may limit the type of material that may be used for the electrodes.

The separator 110 separates the two half-cells 106, 108. In at leastsome embodiments, the separator 110 allows the transport of selectedions (for example, H⁺, Cl⁻, or iron or chromium ions or any combinationthereof) during the charging or discharging of the redox flow batterysystem 100. In some embodiments, the separator 110 is a microporousmembrane. Any suitable separator 110 can be used and examples ofsuitable separator include, but are not limited to, ion transfermembranes, anionic transfer membranes, cationic transfer membranes,microporous separators, or the like or any combination thereof.

Redox flow battery systems can be safe, reliable, and provide a reusableenergy storage medium. It has been challenging, however, to identify aredox flow battery system that has a desirable storage energy with along life (e.g., a flow battery system that maintains its storagecapacity for many charge/discharge cycles), and is made of materialsthat have abundant availability (e.g., materials that are abundant onEarth and are commercially mined and available in relatively largequantities). Current lithium and vanadium batteries utilize materialsthat have limited availability. The storage capacity of manyconventional battery systems also degrades when subjected 10, 50, or 100charge/discharge cycles or more. A further challenge for aqueous redoxflow battery systems is to manage or avoid the evolution of hydrogen oroxygen from water.

As described herein, a suitable and useful redox flow battery system isan iron-chromium (Fe—Cr) redox flow battery system utilizing Fe³⁺/Fe²⁺and Cr³⁺/Cr²⁺ redox chemistry. Iron and chromium are generally readilycommercially available and, at least in some embodiments, the storagecapacity of a Fe—Cr redox flow battery system does not degrade by morethan 10% or 20% over at least 100, 200, 250, or 500 charge/dischargecycles or can be configured, using maintenance procedures, to maintainat least 70%, 80%, or 90% storage capacity over at least 100, 200, 250,or 500 charge/discharge cycles.

In at least some embodiments, the electrolytes (i.e., the catholyte oranolyte) of a Fe—Cr redox flow battery system include an iron-containingcompound or a chromium-containing compound (or both) dissolved in asolvent. In some embodiments, the anolyte and catholyte contain both theiron-containing compound and the chromium-containing compound. Theconcentrations of these two compounds in the anolyte and catholyte canbe the same or different. In other embodiments, the catholyte includesonly the iron-containing compound and the anolyte includes only thechromium-containing compound.

The iron-containing compound can be, for example, iron chloride, ironsulfate, iron bromide, or the like or any combination thereof. Thechromium-containing compound can be, for example, chromium chloride,chromium sulfate, chromium bromide, or the like or any combinationthereof. The solvent can be water; an aqueous acid, such as,hydrochloric acid, hydrobromic acid, sulfuric acid, or the like. In atleast some embodiments, both the catholyte and the anolyte of an Fe—Crredox flow battery system includes iron chloride and chromium chloridedissolved in hydrochloric acid. In at least some embodiments, thecatholyte of an Fe—Cr redox flow battery system includes iron chloridedissolved in hydrochloric acid and the anolyte includes chromiumchloride dissolved in hydrochloric acid.

In at least some instances, it has been found that chloride-complexedchromium ions (for example, Cr(H₂O)₅Cl^(2+/+)) have faster reactionkinetics and lower H₂ production than at least some other chromium ioncomplexes (for example, Cr(H₂O)₆ ^(3+/2+)). Accordingly, the inclusionof chloride in the anolyte (for example, from the chromium-containingcompound, the solvent, or both) can be beneficial.

In at least some embodiments, the molarity of iron in the catholyte orthe anolyte or both is in a range of 0.5 to 2. In at least someembodiments, the molarity of chromium in the anolyte or the catholyte orboth is in a range of 0.5 to 2. In at least some embodiments, themolarity of the hydrochloric acid or other aqueous acid or base is in arange of 0.5 to 4.

One challenge of previous Fe—Cr redox flow batteries is the generationor evolution of hydrogen (H₂) at the negative electrode as a result ofthe redox reactions. In at least some instances, increasing theutilization of the chromium in the redox flow battery can increase theproduction of hydrogen. It is often desirable to limit or reduce theproduction of hydrogen in the redox flow battery.

It has been found that limiting the utilization of chromium results inlower hydrogen generation while retaining adequate energy density in theredox flow battery system. In at least some embodiments, the chromiumutilization in the anolyte of the redox flow battery system is limitedto no more than 80%, 70%, or 60% or less. In at least some embodiments,the chromium utilization in the anolyte is limited by amount of iron inthe catholyte or is limited by 100% utilization of the iron in thecatholyte.

Chromium utilization can be managed, at least in part, by managing therelative amounts of chromium and iron in the redox flow battery system.The term “molar ratio” as used herein means the ratio of the molaramount of one component with respect to the molar amount of a secondcomponent. In at least some embodiments, the molar ratio of chromium inthe anolyte to iron in the catholyte (Cr(anolyte)/Fe(catholyte)) is not1, but, instead, the Cr(anolyte)/Fe(catholyte) molar ratio is at least1.25 or more (for example, at least 1.43, 1.67, or more). In at leastsome embodiments, the molar amount of iron in the catholyte is no morethan 80%, 70%, or 60% or less of the molar amount of chromium in theanolyte. In at least some embodiments, the smaller amount of availableiron limits the utilization of the available chromium to no more than80%, 70%, or 60%. In at least some embodiments, the anolyte and thecatholyte are both mixed iron/chromium solutions.

In at least some embodiments, the concentration of iron in the catholyteis different from the concentration of chromium in the anolyte toproduce the desired molar ratio. In at least some embodiments, theconcentration of iron in the catholyte is no more than 80%, 70%, or 60%or less of the concentration of chromium in the anolyte.

In at least some embodiments, the concentration of iron in the catholyteand the concentration of chromium in the anolyte is the same. In suchembodiments, the molar ratio of chromium and iron in the anolyte andcatholyte, respectively, can be selected by selection of the volumes ofthe anolyte and catholyte. In at least some embodiments, the volumeratio of anolyte to catholyte is at least 1.25:1 or more (for example,at least 1.43:1 or 1.67:1 or more) leading to a molar ratio that isequal to the volume ratio when the concentrations of chromium in theanolyte and iron in the catholyte are the same. In at least someembodiments, the volume of the catholyte is no more than 80%, 70%, or60% of the volume of the anolyte.

In some embodiments, the volumes of the anolyte and the catholyte can bebased on the volume of the respective half-cells 106, 108. In someembodiments, the volumes of the anolyte and the catholyte can be basedon the volume of the respective catholyte and anolyte portions of theredox flow battery system 100. For example, the catholyte portion caninclude the half-cell 108, the catholyte tank 118, and the catholytedistribution arrangement 126. The anolyte portion can include thehalf-cell 106, the anolyte tank 116, and the anolyte distributionarrangement 124.

It will be recognized that a combination of both different iron andchromium concentrations and different catholyte and anolyte volumes canbe used to achieve the desired molar ratio of chromium in the anolyteand iron in the catholyte. In at least some of these embodiments, thevolume of the catholyte is no more than 95%, 90%, 80%, 70%, or 60% ofthe volume of the anolyte.

In at least some instances, it is found that higher H⁺ concentration inthe anolyte promotes hydrogen generation. To reduce hydrogen generationby the anolyte, the H⁺ concentration in the initial anolyte can be lowerthan the H⁺ concentration in the initial catholyte. In at least someembodiments, the H⁺ concentration in the initial anolyte is at least 10,20, 25, or 50 percent lower than the H⁺ concentration in the initialcatholyte.

Table 1 illustrates a 1:1 volume ratio of anolyte to catholyte atdifferent states of charge (SOC) where the state of charge representsthe percentage conversion of the initial active ionic species in theanolyte and catholyte to the reduced/oxidized ionic species. It will berecognized that the concentration of H⁺ changes to maintain chargebalance between the anolyte and catholyte. In Table 1, the initialanolyte is 1.25M Fe²⁺, 1.25M Cr³⁺, and 1.25M H⁺ and the initialcatholyte is 1.25M Fe²⁺, 1.25M Cr³⁺, and 2.5M H⁺. These particularconcentrations are selected so that the H⁺ concentration is equal at the50% state of charge.

TABLE 1 State of Anolyte Catholyte Charge Cr²⁺ Cr³⁺ H⁺ Fe²⁺ Fe³⁺ H⁺ 0 01.25 1.25 1.25 0 2.5 25 0.3125 0.9375 1.5625 0.9375 0.3124 2.1875 500.625 0.625 1.875 0.625 0.625 1.875 75 0.9375 0.3125 2.1875 0.31250.9375 1.5625 100 1.25 0 2.5 0 1.25 1.25

Table 2 illustrates a 2:1 volume ratio of anolyte to catholyte atdifferent states of charge (SOC). In Table 2, the initial anolyte is1.25M Fe²⁺, 1.25M Cr³⁺, and 1.5625M H⁺ and the initial catholyte is1.25M Fe²⁺, 1.25M Cr³⁺, and 2.5M H⁺. These particular concentrations areselected so that the H⁺ concentration is equal when the anolyte is at25% SOC and the catholyte is at 50% SOC. The difference in SOC betweenthe anolyte and catholyte arises due to anolyte having twice the volumeof the catholyte.

TABLE 2 State of Anolyte Catholyte Charge Cr²⁺ Cr³⁺ H⁺ Fe²⁺ Fe³⁺ H⁺ 0 01.25 1.5625 1.25 0 2.5 25 0.3125 0.9375 1.875 0.9375 0.3124 2.1875 500.625 0.625 2.1875 0.625 0.625 1.875 75 0.3125 0.9375 1.5625 100 0 1.251.25

Another challenge with Fe—Cr redox flow battery systems, as well asother redox flow battery systems, is the presence of metal impurities,such as nickel, antimony, and copper. In at least some instances, thesemetal impurities can increase hydrogen generation on the negativeelectrode surface. Such metallic impurities can be present as a naturalimpurity or as a part of the refining or manufacturing of the iron andchromium compounds or other portions of the redox flow battery system orthrough any other mechanism.

In at least some embodiments, the redox flow battery system 100 can beconfigured to remove, or reduce the level of, these impurities. Asillustrated in FIG. 3, in at least some embodiments, to remove, orreduce the level of, these impurities, the redox flow battery system 100is configured to electrochemically reduce at least some of theimpurities to metal form (step 350), collect the resulting metallicparticles using a particulate filter or other arrangement such at theinterdigitated electrode described below (step 352), and remove theseimpurities using a cleaning solution containing an oxidizing species(step 354).

In at least some embodiments, the impurities are reduced within theanolyte as part of the redox reactions. The impurities form metallicparticles or particulates when reduced during charging. The redox flowbattery system 100 may include a particulate filter in the half-cell 106or elsewhere to capture the metallic particles or particulates. In someembodiments, the negative electrode 102 may aid in filtering themetallic particles or particulates. To also facilitate the removal ofthe impurities, the negative electrode 102 can have an interdigitatedstructure, as illustrated in FIG. 2. The interdigitated structureincludes empty or indented channels 240 for collection of particles ofthe metallic impurities during operation of the redox flow batterysystem 100. These particles can then be removed from the electrodeduring a maintenance cycle, as described below.

In at least some embodiments, the Fe—Cr redox flow battery systemsdescribed herein are arranged to remove these impurities using asolution with an oxidizing species, such as Fe³⁺. As part of themaintenance of the redox flow battery system 100, during a maintenancecycle, a Fe³⁺ (or other oxidizing) solution can be flowed through theanolyte portion of the system to remove the impurities from theelectrode 102 or elsewhere in the system. In at least some embodiments,the Fe³⁺ solution can be the catholyte or a portion of the catholyte.Alternative oxidizing solutions include, but are not limited to,hydrogen peroxide solutions, ferric chloride solutions, nitric acid, orthe like.

In at least some embodiments, the removal or reduction of metallicimpurities is performed during manufacturing of the redox flow batterysystem, prior to the onset of operation of the redox flow batterysystem, or during operation of the redox flow battery system, or anycombination thereof. It will be understood that these methods andsystems for removal of metallic impurities are not limited to Fe—Crredox flow battery systems, but can also be utilized in other redox flowbattery systems such as vanadium, vanadium-bromine, vanadium-iron,zinc-bromine, and organic redox flow battery systems.

It has also been found that, in at least some embodiments, occasionalexposure of the electrode 102 to the catholyte 114 can facilitatepassivation of the surface of the electrode 102 and reduce hydrogengeneration. As an example, in one Fe—Cr redox flow battery system theelectrode 102 was treated with the catholyte 114 for 1 hour after 17charge/discharge cycles and the hydrogen generation rate when down from38.9 ml/min to 10.2 ml/min. In at least some embodiments, operation ofthe redox flow battery system can periodically (or when initiated orrequested by an operator) include a maintenance period in which thehalf-cell 106 or electrode 102 is exposed to the catholyte (or anelectrolyte that has components such as those specified above for thecatholyte) for a period of time (for example, 5, 10, 15, 30, 45, 60minutes or more.) The catholyte may be introduced to the half-cell 106or electrode 102 once, periodically, intermittently, or continuouslyduring the maintenance period. In at least some of these embodiments,the catholyte 114 can be returned to the catholyte tank 118 after themaintenance period. In at least some embodiments, the maintenance periodmay be performed when the state of charge of the anolyte is at least50%, 75% or 90%.

FIG. 4 illustrates one embodiment of a redox flow battery system thatincludes switches 434 for disconnecting the anolyte distribution system124 from the half cell 106 and connecting the catholyte distributionsystem 126 to the half-cell 106 to flow catholyte 114 into the half-cell106. Such an arrangement can be used to reduce or remove metallicimpurities or to passivate the electrode 102 or any combination thereof.The pump 122 can be used to flow catholyte 114 into the half-cell 106 orto remove the catholyte 114 from the half-cell 106 when the maintenanceis complete.

A Fe—Cr redox flow battery system can have a reduction in storagecapacity over time arising, at least in part, from the low standardpotential of the Cr²⁺/Cr³⁺ pair which results in at least some level ofhydrogen generation on the anolyte side of the system. As a result theAverage Oxidation State (AOS) of the active species in the systemincreases and the system can become unbalanced and the storage capacitydecrease. It is useful, therefore, to have methods or arrangements forat least partially restoring the storage capacity by recovering the AOS.

In at least some embodiments, the AOS for a Fe—Cr redox flow batterysystem can be described as: AOS=((Moles of Fe⁺ in catholyte andanolyte)*3+(Moles of Fe²⁺ in catholyte and anolyte)*2+(Moles of Cr⁺ inanolyte and catholyte)*3+(Moles of Cr²⁺ in anolyte andcatholyte)*2)/(Moles of Fe in catholyte and anolyte+Moles of Cr inanolyte and catholyte).

To rebalance the redox flow battery system, in at least someembodiments, the redox flow battery system includes a balancearrangement, in conjunction with either the anolyte or catholyte, torebalance the system and restore storage capacity. In at least someembodiments, the balance arrangement utilizes a vanadium source (toproduce oxovanadium (VO²⁺) and dioxovanadium (VO₂ ⁺) ionic species) anda reductant, such as an oxidizable hydrocarbon compound, to rebalancethe system and restore storage capacity. The following embodimentsillustrate the addition of a balance arrangement to a Fe—Cr redox flowbattery system. It will be understood that such balance arrangements canbe used with other redox flow battery systems, or other chemical and/orelectrochemical systems.

FIG. 5A illustrates one embodiment of portions of the redox flow batterysystem 100 and a balance arrangement 500. FIG. 5B illustrates oneembodiment of the balance arrangement 500. In this embodiment, thecatholyte 114 is used in conjunction with a balancing electrolyte 562(for example, an electrolyte containing VO²⁺/VO₂ ⁺) and a reductant 563to rebalance the redox flow battery system 100. The balance arrangement500 includes the catholyte tank 118; balance electrodes 552, 554;balance half-cells 556, 558; balance separator 560; catholyte balancepump 572; catholyte balance distribution system 576; balance tank 566;reductant tank 567; balance electrolyte pump 570; balance electrolytedistribution arrangement 574; and potential source 561.

The following reaction equations illustrate one example of therebalancing of the system using the iron-based catholyte 114, abalancing electrolyte 562 containing oxovanadium ions, and a reductant563 containing fructose, along with the application of an externalpotential from the potential source 561 of at least 0.23 V:

VO²⁺+H₂O+Fe³⁺→VO₂ ⁺+Fe²⁺+2H⁺

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O

Via these reactions, the AOS of the redox flow battery system 100 can bereduced and the H⁺ ions lost in hydrogen generation restored. In atleast some embodiments, this rebalancing (or restoring of the AOS orstorage capacity recovery) does not utilize any metallic catalyst assuch catalysts often increase hydrogen generation. In at least someembodiments, VO²⁺ of the balance electrolyte 562 can be considered ahomogeneous catalyst as the VO²⁺ ions are regenerated using thereductant 563. In at least some embodiments, the reduction of VO₂ ⁺ ionshappens in balance half cell 566.

In at least some embodiments, the oxidation of the reductant 563 can beperformed in the balance tank 566 instead of the half-cell 556 and maynot require the application of an external potential, as long as VO₂ ⁺ions are available. Suitable reducing agents include sugars (forexample, fructose, glucose, sucrose, or the like or any combinationthereof), carboxylic acids (for example, formic acid, acetic acid,propionic acid, oxalic acid, or the like or any combination thereof),aldehydes (for example, formaldehyde, acetaldehyde, or the like or anycombination thereof), alcohols (for example, methanol, ethanol,propanol, or the like or any combination thereof), other hydrocarbons,or hydrogen gas. In at least some embodiments, the reductant is solubleor at least partially soluble in water.

In at least some embodiments, the reductant 563 is added eitherperiodically, intermittently, or continuously to the balance electrolyte562 from the reductant tank 567. In at least some embodiments, thisrebalancing process (for recovering the storage capacity or restoringthe AOC) occurs continuously, intermittently, or periodically. Forexample, the catholyte balance pump 572 and balance electrolyte pump 570can operate continuously, intermittently, or periodically. In at leastsome embodiments, the catholyte pump 122 can also be used as thecatholyte balance pump 572. Moreover, the catholyte balance distributionarrangement 576 may include a valve to couple to, or disconnect from,the catholyte tank 118.

FIGS. 5C and 5D illustrate another embodiment of redox flow batterysystem 100 with a balance arrangement 500′ which operates with theanolyte 112 (and corresponding anolyte pump 572′ and anolyte balancedistribution arrangement 576′) instead of the catholyte. In at leastsome embodiments, the anolyte pump 120 can also be used as the anolytebalance pump 572′.

The following reaction equations illustrate one example of therebalancing of the system using the chromium-based anolyte 112, abalancing electrolyte 562 containing oxovanadium ions, and a reductant563 containing fructose, along with the application of an externalpotential from the potential source 561 of at least 1.40 V:

VO²⁺+H₂O+Cr³⁺→VO₂ ⁺+Cr²⁺+2H⁺

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O

FIG. 5E illustrates another embodiment of a balance arrangement 500″which can be adapted to operate with either the catholyte or anolyte andthe corresponding catholyte/anolyte tank 118/116 that is coupled to theremainder of the redox flow battery system 100. This embodimentincorporates an intermediate tank 584 and two intermediate half-cells586, 588 between the catholyte/anolyte tank 118/116 and the balance tank562 and corresponding half-cells 556/558. (As with the balance tank,there can be an intermediate pump and intermediate distributionarrangement, as well as an intermediate separator between the twohalf-cells 586, 588 and a source potential to apply a potential betweenthe electrodes of the two half-cells 586, 588.) In one embodiment, theintermediate electrolyte in the intermediate tank 584 contains V²⁺/V³⁺ions.

The following reaction equations illustrate one example of therebalancing of the system using balance arrangement 500″ and thecatholyte 114 of redox flow battery system 100 (FIG. 1).

VO²⁺+H₂O−e⁻→VO₂ ⁺+2H⁺ (half-cell 556)

V³⁺+e⁻→V²⁺ (half-cell 558)

V²⁺−e⁻→V³⁺ (half-cell 586)

Fe³⁺+e⁻→Fe²⁺ (half-cell 588)

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O (balance tank 562 or half cell556 or both)

Another embodiment uses the anolyte (Cr²⁺/Cr³⁺) instead of the catholytein conjunction with the intermediate electrolyte and balanceelectrolyte. Yet another embodiment uses the anolyte and replaces theV²⁺/V³⁺ intermediate electrolyte with a Fe²⁺/Fe³⁺ intermediateelectrolyte.

It will be recognized that the balance arrangement described herein canbe utilized with other redox flow battery systems and, in particular,those that are capable of generating hydrogen gas. Examples of suchredox flow battery system include, but are not limited to, Zn—Br orZn-Cl redox flow battery systems, vanadium-based (for example, allvanadium, V—Br, V—Cl, or V-polyhalide) redox flow battery systems; Fe—Vor other iron-based redox flow battery systems (for example, an all ironredox flow battery system); or organic redox flow battery systems.

In some embodiments, during Fe²⁺-overcharging conditions, chlorine gas(Cl₂) can be generated on the catholyte side of the redox flow batterysystem 100. The chlorine may be confined in the catholyte headspace of,for example, the catholyte tank 118 or half-cell 108 or the like or anycombination thereof. Continued generation of chlorine gas increases thepressure in the confined catholyte headspace. In at least someembodiments, this may result in the chlorine gas migrating to theanolyte headspace via a connection 638 c (FIG. 6C) which optionallyincludes one or more valves or switches 639 to control flow. In at leastsome embodiments, at least a portion of the chlorine gas may be absorbedby the anolyte solution. In at least some embodiments, the followingreactions can occur between chlorine and the anolyte solution tochemically discharge the over-charged system:

2Cr²⁺+Cl₂→2Cr³⁺+2Cl⁻

2Fe²⁺+Cl₂→2Fe³⁺+2Cl⁻

In at least some embodiments, the redox flow battery system 100 mayinclude a pressure release system to manage pressure in the catholyte oranolyte headspace. For example, a pressure relief valve 638 a (FIG. 6A)or a liquid-containing U-tube arrangement 638 b (FIG. 6B) may be coupledto the catholyte headspace to manage the pressure. Similarly, a pressurerelief valve or a liquid-containing U-tube arrangement may be coupled tothe anolyte headspace. In at least some embodiments, gas in the anolyteor catholyte headspace may exchange with an environmental atmosphere viaa bi-directional gas pressure control system such as the U-tubearrangement. In at least some embodiments, a U-tube arrangement may alsobe used as a gas leak monitor. In at least some embodiments, the liquidin a U-tube arrangement may contain an acid level indicator that can beused to estimate the amount of acid-containing gas released into theenvironment by the redox flow battery system.

In at least some instances, the acidic solutions and chemical vapor fromleaks of the electrolytes and chemical products of the redox reactionscan damage electronic devices (for example, the controller 128,switches, valves, pumps, sensors, or the like) in the redox flow batterysystem 100. In addition, the leaks may result in environmental damage orcontamination.

In at least some embodiments, all or a portion of the redox flow batterysystem 100 that contains the anolyte or catholyte or both can besituated in a secondary container 790 (FIG. 7) that contains acidabsorbent material, such as sodium carbonate, sodium bicarbonate,calcium carbonate, or calcium oxide or the like. In at least someembodiments, the secondary container can contain sufficient acidabsorbent material to neutralize at least 10, 25, 40, 50, 60, 70, 75, 90percent or more of the anolyte or catholyte or both.

In some embodiments, the anolyte and catholyte containing components,such as the anolyte or catholyte tanks 116, 118, half-cells 106, 108, atleast some portions of the anolyte or catholyte distribution systems124, 126, electrodes 102, 104, or the like, of the redox flow batterysystem 100 are maintained at a temperature of at least 50, 60, 70, or 80degrees Celsius or more during charge or discharge periods in atemperature zone 892, as illustrated in FIG. 8. The temperature of thesecomponents may be maintained using one or more heating devices 894. Inaddition, one or more of electronic components of the redox flow batterysystem, such as one or more of the controller 128, the pumps 120, 122,one or more sensors, one or more valves, or the like, are maintained ata temperature of no more than 40, 35, 30, 25, or 20 degrees Celsius orless. The temperature of these components may be maintained using one ormore cooling devices 896.

In a redox flow battery system, the Average Oxidation State (AOS) of theactive species in the catholyte or the anolyte (or both) can change,particularly under conditions such as hydrogen generation or oxygenintrusion in the system. As a result of the AOS change, the system canbecome unbalanced, the system storage capacity may decrease, or sidereactions, such as hydrogen generation, may be accelerated, or anycombination of these effects.

It is useful to know the AOS of a redox flow battery system.Conventional AOS determination methods include taking samples andconducting an off-line potential titration analysis, obtaining in situUV-Vis measurement; or performing in situ potential differencemeasurements against a reference electrode. These techniques can be slowor may be relatively inaccurate.

In contrast to these conventional techniques, relatively fast andaccurate methods are presented herein. The method includes measuring thecapacity of ions in at least one of the electrolytes during a lowpotential charging process and using this charging capacity and theknown volume of the electrolyte to determine the AOS.

FIG. 9 illustrates three different charging curves for a redox flowbattery system. The first charging curve 990 is for a balanced system.The second charging curve 992 and third charging curves 994 are for twodifferent un-balanced systems. In the unbalanced systems, there is a lowpotential charging region 992 a, 994 a and a high potential chargingregion 992 b, 994 b. By performing one or more measurements in the lowpotential charging region 992 a, 994 a, the AOS can be determined.

This AOS determination methods will be illustrated using the Fe—Cr redoxflow battery system described above. It will be understood, however,that these methods can be used with any other suitable redox flowbattery system. In this example, both the catholyte and the anolyteinclude iron and chromium ions. For a balanced system, when the redoxflow battery system is fully discharged, there is only Fe²⁺ and Cr³⁺ inboth the anolyte and catholyte. Upon application of a charge, the systemundergoes the following redox reactions:

Fe²⁺⁻→Fe³⁺+e (E°=+0.77 V, positive side)

Cr³++e⁻→Cr²⁺ (E°=−0.40 V, negative side)

The potential difference between the positive and negative electrolytescan increase to more than 0.9 V immediately in at least someembodiments, as illustrated by first charging curve 990 in FIG. 9.

If the system is unbalanced, however, AOS increases due to sidereactions. For example, a mixture of Fe²⁺/Fe³⁺/Cr³⁺ can be found in thedischarged electrolyte of an unbalanced Fe—Cr redox flow battery system.For such a mixture of ions, when a charge is applied to the redox flowbattery system, the system first undergoes the following redoxreactions:

Fe²⁺⁻→Fe³⁺+e (positive side)

Fe³⁺+e⁻→Fe²⁺(negative side)

These reactions occur at low potential until all of the Fe³⁺ ions areconsumed at the negative electrolyte. This corresponds to the lowpotential charging region 992 a, 994 a in FIG. 9. Only after consumingall of the Fe³⁺ ions will the chromium ions be reduced at the highercharging potential, as illustrated in high potential charging regions992 b, 994 b.

The charge capacity of this low potential charging process can be usedto determine the amount of Fe³⁺ ions in the anolyte. For example, thecharge capacity at the turning point 993 of FIG. 9 divided by 26.8Ah/mole (the charge on one mole of electrons) gives the molar amount ofFe³⁺ in the anolyte. This combined with the volume of the positive andnegative electrolytes can be used to determine the AOS of the redox flowbattery system.

This information can also be used to rebalance the redox flow batterysystem using, for example, the balance arrangements described above. Asan example, using the balance arrangement 500 illustrated in FIGS. 5Aand 5B, a balancing electrolyte 562 containing oxovanadium ions, and areductant 563 containing fructose, along with the application of anexternal potential from the potential source 561 of at least 0.23 Vproduces the following reactions:

VO²⁺+H₂O+Fe³⁺→VO₂ ⁺+Fe²⁺+2H+

24VO₂ ⁺+24H⁺+C₆H₁₂O₆→24VO²⁺+6CO₂+18H₂O

Rebalancing the AOS can be accomplished by providing the reductant 563(fructose) to the balancing electrolyte 562. For example, ideallyproviding a molar amount of fructose equal to the molar amount of Fe³⁺multiplied by (1/24) can rebalance the AOS. It will be recognized thatmore fructose may be used to fully rebalance the AOS due to thenon-ideal elements in the system.

FIG. 10 is a flowchart of one embodiment of a method of determining AOS.In step 1060, the charge capacity during a low potential charging periodis measured. For example, the charge capacity can be measured as thecharge capacity when the charging curve changes from the low potentialcharging period to the high potential charging period, such as, forexample, at turning points 993, 995 in FIG. 9. In step 1062, the AOS canthen be determined using this measurement (or set of measurements) andthe known volumes of the anolyte and catholyte and the known iron andchromium concentrations of the catholyte and anolyte, respectively. Inat least some embodiments, prior to step 1060, the anolyte 112 andcatholyte 114 in the redox flow battery system 100 can be fullydischarged by applying an external potential or by complete mixing ofthe anolyte and catholyte. In at least some embodiments, the catholyteand anolyte may be fully mixed prior to determining the charge capacity.

Another embodiment does not require a fully discharged system, but canbe determined for any state-of-charge condition using columbictitration. FIG. 11 is a flowchart of one embodiment of this method. Instep 1166, the amount of one ionic species (for example, Fe³⁺ or Cr³⁺)is measured in one of the electrolytes (catholyte or anolyte) by haltingthe flow of that electrolyte and performing in situ titration of thefirst ionic species in the non-flowing electrolyte. The balancearrangement 500 or the other electrolyte, which is preferably continuingto flow through the redox flow battery system, can be used to titratethe ionic species. As an example, the amount of Fe³⁺ can be determinedin the catholyte by halting flow of the catholyte and titrating the Fe³⁺using either a) the vanadium ions in the balance arrangement 500 or b)the Cr²⁺ in the anolyte, which may continue to flow through the redoxbattery system in order to ensure that there is sufficient Cr²⁺ ions totitrate all of the Fe^(3+.)

In step 1168, the amount of a second ionic species (for example, Cr²⁺ orFe²⁺) is measured in one of the electrolytes (anolyte or catholyte) byhalting the flow of that electrolyte and performing in situ titration ofthe second ionic species in the non-flowing electrolyte. Preferably, thesecond ionic species is measured in a different electrolyte from theelectrolyte in which the first ionic species is measured so that oneionic species is measured in the catholyte and one ionic species ismeasured in the anolyte. In at least some embodiments, the measurementof the first ionic species will alter the amount or concentration of thesecond ionic species so that the measurement of the second ionic specieswill be adjusted to take into account this alteration. The balancearrangement 500 or the other electrolyte, which is preferably continuingto flow through the redox flow battery system, can be used to titratethe ionic species. As an example, the amount of Cr²⁺ can be determinedin the anolyte by halting flow of the anolyte and titrating the Cr²⁺using either a) the vanadium ions in the balance arrangement 500 or b)the Fe³⁺ in the catholyte, which may continue to flow through the redoxbattery system in order to ensure that there is sufficient Fe³⁺ ions totitrate all of the Cr²⁺.

In step 1170, the AOS can then be determined using these twomeasurements, the initial concentrations or amounts of iron andchromium, the volumes of the half-cells, and the volumes of the anolyteand catholyte.

In another embodiment, the discharge process, instead of the chargingprocess, can be observed. For a charged or partly charged system, adischarge or self-discharge process can be used to estimate the extraFe³⁺ in the positive electrolyte based on the discharge rate differencebetween two different electrochemical pairs (here, Fe²⁺/Fe³⁺ vsCr²⁺/Cr³⁺) and one pair with different concentrations (Fe²⁺/Fe³⁺). Thechange in discharge rate for the two electrochemical pairs in anelectrochemical device is much faster than that of one electrochemicalpair with different concentrations. As a result, as illustrated in FIG.12 a turning point 1297 can be observed in the discharge curve 1296. Inat least some embodiments, this turning point 1297 can be used toestimate the amount of extra Fe³⁺ in the catholyte. FIG. 13 is oneembodiment of a method of determining the AOS using the discharge curve.In step 1374, the discharge rate is measured during initial discharge.In step 1376, a turning point in the voltage discharge is determined andan open circuit voltage is measured. In step 1378, the open circuitvoltage after the turning point can be used to estimate the extraconcentration of the active electrochemical pair (here, Fe³⁺/Fe²⁺⁺)using the Nernst equation. In at least some embodiments, the dischargeend point is selected to be the point at which there is no more than 1%,5%, or 10% Cr²⁺ ions (of the total chromium) in the anolyte. In step1380, the AOS can be determined from the estimated concentration, andknown volumes of the electrolytes. This is the reverse process of thepreviously described method illustrated in FIG. 10.

In at least some embodiments, the methods of determining AOS describedabove and illustrated using FIGS. 9 to 13 can be performed in situ usingthe half-cells 106, 108, anolyte 112, catholyte 114, other elements ofthe redox flow battery system 100, or elements of the balance system500. In other embodiments, the methods of determining AOS may includeflowing a portion of the anolyte 112 or catholyte 114 or both into oneor more other half-cells for measurements. In yet other embodiments, themethods of determining AOS can include removing portions of the anolyte112 or catholyte 114 or both and performing measurements external to theredox flow battery system 100.

In at least some embodiments, the determined AOS can be used to estimatethe amount of hydrogen generated or the production of other sideproducts. In at least some embodiments, the determination of AOS inFIGS. 10, 11, and 13 can be followed by operation of a balancingarrangement, as described above, to rebalance the redox flow batterysystem and restore the AOS. Such operation can include, for example,determining an amount of the reductant 563 to add to the balancingelectrolyte 562.

For a given redox flow battery system, there is a fixed ratio of theelectrolyte volume inside the battery stack and the whole batterysystem. The electrolyte volume inside the battery stack is always muchsmaller than that outside the electrolyte tanks. Thus, it is muchquicker to charge and discharge the electrolyte inside the stack than tocharge and discharge the electrolyte in the whole system. The solutionfor a quick measurement for the available capacity of the redox flowbattery is the charge or discharge of the electrolyte in the stack onlywithout electrolyte flow at a given operation condition and then convertthe result to the whole system based on the system design parameters.

There is often a desire to know the available energy or storagecapacity, or a change in that energy or storage capacity, of a redoxflow battery system. Conventional methods for such determinationinclude, for example, taking samples from the redox flow battery systemto conduct off-line titration or other analysis, in situultraviolet-visible (UV-Vis) measurements, or in situ potentialdifference measurements against a reference electrode. These methods canbe slow or inaccurate.

It has been found that, during the operation of a redox flow batterysystem, when the active material become unbalanced, the end OCV (opencircuit voltage) of the redox flow battery system for a given set ofdischarge conditions changes. The difference in the end OCV has a directrelationship with its usable storage or charge capacity. In addition,the end OCV can have a direct relationship with the AOS. In particular,for a given redox flow battery system, under the same discharge rate,there is a reliable relationship between the end OCV and the systemstorage or charge capacity or AOS. Under certain conditions, such as H₂generation in the anolyte side of the system or O₂ intrusion into thesystem, the active species in the system become unbalanced. As a result,the system storage or charge capacity decreases and side reactions arefurther accelerated. The relationship between the OCV and the systemstorage or charge capacity or AOS also changes.

As an example, for one embodiment of a Fe—Cr redox flow battery system,after being charged to an OCV of 1.100V and this discharged at 33 mW/cm²to 0.60V, the discharge energy and related end OCV are given in thetable below.

End OCV (V) 0.874 0.872 0.865 0.860 0.854 0.850 0.848 Discharge Energy(Wh) 890 900 926 949 968 986 1001As shown in this table, the end OCV under the same discharge conditionscan be used as an indicator for the available storage or charge capacityof the system. Accordingly, the end OCV can also be used as an indicatorof the AOS.

Thus, in at least some embodiments, the available storage or chargecapacity or the AOS can be determined by total discharge of the systemunder a set of given conditions, such as at a preselected discharge rateor power, followed by measurement of the end OCV. This end OCV can becompared to a pre-determined OCV curve; a pre-determined look-up tableor other calibration table, chart, or the like; or applied to apre-determined mathematical relationship to determine the storage orcharge capacity or the AOS of the redox flow battery system.

In at least some embodiments, the discharge and end OCV measurement canbe performed using the half-cells 106, 108 and electrodes 102, 104. Inat least some embodiments, the entire redox flow battery system isdischarged.

For a redox flow battery system, there is typically a fixed ratio of theelectrolyte volume in the battery stack (e.g., half-cells 106, 108) andthe whole system. The electrolyte volume inside the battery stack istypically much smaller than the volume in the electrolyte tanks 116,118. Thus, it can be much quicker to charge and discharge theelectrolyte inside the half-cells 106, 108 than to charge and dischargethe electrolyte in the whole system. Accordingly, in at least someembodiments, flow of the electrolytes (anolyte and catholyte) may behalted during this storage capacity determination so that only theelectrolyte in the half-cells 106, 108 is discharged. Typically, the endOCV measurement of the electrolytes in the half-cells 106, 108 isindicative of the entire redox flow battery system as a whole.

Moreover, it may be advantageous to use a cell with even smaller volumethan the half-cells of the battery stack (for example, half-cells 106,108) to perform the discharge and measurement of end OCV. In at leastsome embodiments, the redox flow battery system 100 can include an OCVcell 1401 with half-cells 1406, 1408 and electrodes 1402, 1404, asillustrated in FIG. 14. In at least some embodiments, the OCV cell 1401may be at least 25%, 30%, 40%, 50%, 60%, or 75% smaller in volume thanthe half-cells 106, 108 and electrodes 102, 104. The OCV cell 1401 maybe in-line with the flow of electrolytes (anolyte and catholyte) throughthe half-cells 106, 108, as illustrated in FIG. 14. Alternatively, theOCV cell 1401 may be positioned outside of the in-line flow of theelectrolytes through the anolyte distribution arrangement 124 andcatholyte distribution arrangement 126 and the OCV cell 1401 can befilled with the electrolytes using valves, switches, or the like. It maybe advantageous to have a relatively small OCV cell 1401 as thedischarge process in the OCV cell may be faster than in the half-cells106, 108 (with the flow of electrolytes halted) resulting in a fastermeasurement of the end OCV and determination of the storage capacity.Accordingly, in at least some embodiments, flow of the electrolytes(anolyte and catholyte) may be halted during this storage capacitydetermination so that only the electrolyte in OCV cell 1401 isdischarged.

FIG. 15 is a flowchart of one embodiment of a method of determiningstorage or charge capacity or AOS of a redox flow battery system. Instep 1574, the redox flow battery system is discharged at a preselecteddischarge rate. In at least some embodiments, the actual discharge ratevaries from the predetermined discharge rate by no more than 1, 5, or10%. In at least some embodiments, the discharge is a self-discharge ofthe redox flow battery system. In at least some embodiments, the enddischarge point is when the amount of Cr²⁺ ions in the anolyte is nomore than 1%, 5%, or 10% of the total chromium. Other end dischargepoints can be used.

In step 1576, after the discharge, the end OCV is measured. In step1578, the measured end OCV is compared to a pre-determined OCV curve; apre-determined look-up table or other calibration table, chart, or thelike; or applied to a pre-determined mathematical relationship thatrelates the end OCV to concentrations of one or more active species, thestorage or charge capacity, or the AOS of the redox flow battery system.In at least some embodiments, a pre-determined end OCV curve; apre-determined look-up table or other calibration table, chart, or thelike; or applied to a pre-determined mathematical relationship isobtained by experimentally measuring the end OCV, using the preselecteddischarge rate, of redox flow battery systems with different values ofconcentrations of one or more active species, storage or chargecapacity, or AOS.

In optional step 1580, when the concentrations of one or more activespecies or the storage or charge capacity is determined in step 1578,the concentrations of one or more active species or storage or chargecapacity can be used to determine the AOS using the electrolyte volumes.

In at least some embodiments, when the storage or charge capacity or AOSis determined and indicates that the system is unbalanced, any of thetechniques described above can be employed to rebalance the redox flowbattery system.

AOS can also be determined using other measurements of OCV. FIG. 16 is aflowchart of one embodiment of a method of determining AOS of a redoxflow battery system. In step 1680, the OCV of a redox flow batterysystem is measured. In step 1682, one or both of the followingprocedures are performed: a) an amount of one iron ionic species (forexample, Fe³⁺ or Fe²⁺) in the catholyte is measured; or b) an amount ofone chromium ionic species (for example, Cr³⁺ or Cr²⁺) in the anolyte ismeasured. It will be recognized that measurement of other ionic speciesin either the anolyte or catholyte (or in other types of redox flowbattery systems) can be used in this step. In at least some embodiments,the two half-cells used for measurement of the amount of the one ironionic species or the one chromium ionic species can be part of thebattery stack of the redox flow battery system, such as half-cells 106,108. In other embodiments, the two half-cells are not part of thebattery stack, but can be, for example, the OCV cell 1401 of FIG. 14. Inat least some embodiments, the measurement is made without electrolyteflow in the redox flow battery system. In at least some embodiments, theflow of the electrolyte that is the object of the measurement is haltedwhile the flow of the other electrolyte is maintained to facilitatecomplete titration of the ionic species being measured. In at least someembodiments, the measurement of the one iron ionic species or the onechromium ionic species can utilize the balance arrangement 500 of FIG.5A and the measurement steps can include the reduction of the one ironionic species or the one chromium ionic species and the oxidation ofvanadium ions followed by the regeneration of the vanadium ions byreducing the dioxovanadium ions using a reductant, as described above.In at least some embodiments, the measurement of the one iron ionicspecies or the one chromium ionic species can be performed off-line orcan be performed in situ.

In step 1684, the AOS is determined using i) the measured amount of theone iron ionic species, the measured amount of the one chromium ionicspecies, or the measured amounts of both of the one iron ionic speciesand the one chromium ionic species, ii) the measured OCV, and iii) arelationship between the amount or concentration of the one iron ionicspecies or the one chromium ionic species in the catholyte or anolyte,respectively, and the OCV. In at least some embodiments, the AOS can bedetermined from a pre-determined OCV curve, look-up table, calibrationtable, or mathematical relationship relating OCV to one of thefollowing: a concentration or amount of the one iron ionic species, aconcentration or amount of the one chromium ionic species, orconcentrations or amounts of both the one iron ionic species or the onechromium ionic species.

In at least some embodiments, the pre-determined OCV curve, look-uptable, calibration table, or mathematical relationship can be arelationship between the OCV and the measured or calculated ionicspecies for a balanced system. The measured OCV can provide an expectedconcentration or amount of that ionic species. The difference between 1)the expected concentration or amount of the measured ionic speciesversus 2) the measured concentration or amount of the ionic species canbe used to determine the AOS.

In at least some embodiments, the pre-determined OCV curve; apre-determined look-up table or other calibration table, chart, or thelike; or applied to a pre-determined mathematical relationship isobtained by experimentally measuring the OCV for different amounts orconcentrations of the selected ionic species in a balanced system.

In at least some embodiments, when the AOS is determined and indicatesthat the system is unbalanced, any of the techniques described above canbe employed to rebalance the redox flow battery system.

The above specification provides a description of the manufacture anduse of the invention. Since many embodiments of the invention can bemade without departing from the spirit and scope of the invention, theinvention also resides in the claims hereinafter appended.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for determining an average oxidationstate (AOS) of a redox flow battery system, the method comprisingmeasuring a charge capacity for a low potential charging period startingfrom a discharged state of the redox flow battery system to a turningpoint of a charge voltage; and determining the AOS using the measuredcharge capacity and volumes of anolyte and catholyte of the redox flowbattery system.
 2. The method of claim 1, wherein the anolyte and thecatholyte of the redox flow battery system both comprises chromium ionsand iron ions and determining the AOS further comprises determining theAOS using initial concentrations of chromium ions and iron ions in theanolyte and catholyte.
 3. The method of claim 2, wherein a molar ratioof chromium in the anolyte to iron in the catholyte is at least 1.25. 4.The method of claim 2, wherein the low potential charging periodcorresponds to a period of reduction of Fe³⁺ in the anolyte.
 5. Themethod of claim 1, further comprising rebalancing the AOS in response tothe determination.
 6. The method of claim 5, wherein rebalancing the AOScomprises oxidizing vanadium ions in a balancing electrolyte todioxovanadium ions to produce hydrogen ions, wherein the anolyte orcatholyte of the redox flow battery system form a balancing arrangementwith the balancing electrolyte using at least two half-cells; andregenerating the vanadium ions by reducing the dioxovanadium ions usinga reductant.
 7. The method of claim 1, further comprising, prior tomeasuring the charge capacity, applying a potential to discharge theredox flow battery system.
 8. The method of claim 1, wherein measuringthe charge capacity comprises measuring the charge capacity in theanolyte within the redox flow battery system.
 9. The method of claim 1,wherein measuring the charge capacity comprises measuring the chargecapacity in a portion of the anolyte removed from the redox flow batterysystem.
 10. A method for determining an average oxidation state (AOS) ofa redox flow battery system comprising a catholyte and an anolyte thatboth comprise iron and chromium ions, the method comprising measuring insitu an amount or concentration of a first ionic species in a first oneof either the catholyte or the anolyte; measuring in situ an amount orconcentration of a second ionic species in a second one of either thecatholyte or the anolyte; and determining the AOS using the measuredamounts or concentrations, initial concentrations or amounts of iron andchromium in the catholyte and anolyte, and volumes of anolyte andcatholyte of the redox flow battery system.
 11. The method of claim 10,wherein a molar ratio of chromium in the anolyte to iron in thecatholyte is at least 1.25.
 12. The method of claim 10, furthercomprising rebalancing the AOS in response to the determination.
 13. Themethod of claim 12, wherein rebalancing the AOS comprises oxidizingvanadium ions in a balancing electrolyte to dioxovanadium ions toproduce hydrogen ions, wherein the anolyte or catholyte of the redoxflow battery system form a balancing arrangement with the balancingelectrolyte using at least two half-cells; and regenerating the vanadiumions by reducing the dioxovanadium ions using a reductant.
 14. Themethod of claim 10, wherein measuring in situ the amount orconcentration of the first ionic species comprises measuring the amountor concentration of the first ionic species with the first one of eitherthe catholyte or the anolyte not flowing and another one of thecatholyte or anolyte flowing.
 15. The method of claim 14, whereinmeasuring in situ the amount or concentration of the second ionicspecies comprises measuring the amount or concentration of the secondionic species with the second one of either the catholyte or the anolytenot flowing and another one of the catholyte or anolyte flowing.
 16. Themethod of claim 10, wherein measuring in situ the amount orconcentration of the first ionic species comprises measuring the amountor concentration of the first ionic species by electrochemicallytitrating the first ionic species using vanadium ions in a balancingelectrolyte.
 17. A method for determining an average oxidation state(AOS) of a redox flow battery system comprising a catholyte and ananolyte that both comprise iron and chromium ions, the method comprisingmeasuring a discharge rate during discharge of a redox flow batterysystem; determining a turning point of the discharge and measure an opencircuit voltage (OCV) after the turning point; estimating aconcentration of Fe³⁺ using the measured OCV; and determining the AOSusing the measured concentration and volumes of anolyte and catholyte ofthe redox flow battery system.
 18. The method of claim 17, wherein amolar ratio of chromium in the anolyte to iron in the catholyte is atleast 1.25.
 19. The method of claim 17, further comprising rebalancingthe AOS in response to the determination.
 20. The method of claim 19,wherein rebalancing the AOS comprises oxidizing vanadium ions in abalancing electrolyte to dioxovanadium ions to produce hydrogen ions,wherein the anolyte or catholyte of the redox flow battery system form abalancing arrangement with the balancing electrolyte using at least twohalf-cells; and regenerating the vanadium ions by reducing thedioxovanadium ions using a reductant.